I've never noticed much recoil difference between the 9mm and 45 for recoil. Yes, the .45 has more but it's not as if one is managible and the other a monster. Definitely not enough difference to knock one backwards vs. the other.

Like I said, it was most likely a combined lack of proper stance, flinch and improper grip. I doubt there's actually that big a difference between the two.

I appreciate all of the advice, I hate living in a state where I can't try before I buy.